Ossification - formation of bone tissue - can occur by two processes: intramenbranous ossification and endochondral ossification.
In the first case, bone tissue gradually appears in a conjunctival, non-cartilaginous membrane.
In endochondral ossification, a bone-shaped cartilage piece serves as a template for making bone tissue. In this case, the cartilage is gradually destroyed and replaced by bone tissue.
Growth in long bones
Endochondral ossification occurs in the formation of long bones, such as the legs and arms.
In these bones, two major regions will undergo ossification: the long cylinder, known as the diaphysis, and the dilated ends, which correspond to the epiphyses.
Between the epiphysis of each extremity and the diaphysis is maintained a region of cartilage, known as growth cartilage, which will allow the constant occurrence of endochondral ossification, leading to the formation of more bone. In this process, osteoclasts play an important role. They constantly effect resorption of bone tissue while new bone tissue is formed.
Osteoclasts act as true bone breakers, while osteoblasts play the role of more bone builders. In this sense, the process of bone growth depends on the joint action of pre-existing bone resorption and the deposition of new bone tissue. Considering, for example, the increase in diameter of a long bone, it is necessary to resorb the inner layer of the bone wall, while depositing more bone in the outer wall.
Growth occurs until a certain age is reached, from which the growth cartilage also ossifies and bone growth in length ceases.